The present invention relates to the seismic prospecting of the sub-surface.
With this technique, a number of seismic sensors as well as one or more sources of artificial seismic shocks are arranged on the soil. In simple versions, the sensors and sources are situated in the same vertical plane. They can be considered as approximately aligned if the unevenness of the terrain is disregarded as the effects thereof can be corrected later on. The seismic sensors and sources are usually also distributed regularly, often at the same intervals.
The various seismic sources are excited successively on the terrain. Each time a source is excited, the various seismic signals or "traces" received by each of the sensors owing to the acoustic waves produced by the shock are recorded selectively. Each trace therefore corresponds to a source-sensor pair.
In so-called "reflection" seismics, one is interested in the reflections of acoustic waves from "reflection points" in the sub-surface. For this purpose, it is known to combine all the traces for which the source and the sensor are symmetrical about a given vertical, for example by adding all these traces. The reflections appearing in the resultant trace indicate the reflection points. Suitable graphic representation of the resultant traces associated with the various verticals permits geophysicists to understand better the structure of the sub-surface. This process of so-called "multiple coverage" reflection seismics therefore seeks reflection points corresponding to the same depth in the various traces received.
Reflection seismics with multiple coverage will give valuable information about the sub-surface. However, "deaf zones" sometimes appear and result in uncertainties about the interpretation of the profile of certain strata. More generally, although reflection seismics defines the strongly reflecting interfaces fairly well, it does not permit the features in the intervals between interfaces of the sub-surface (velocity and absorbtion in particular) to be analysed in detail.